1. Field of the Invention
The invention relates to sol-gel techniques for fabricating silica articles, in particular silica optical fiber.
2. Discussion of the Related Art
Silica optical fiber is produced from a glass preform, the preform typically consisting of a doped silica core surrounded by one or more claddings. As discussed in F. DiMarcello et al. xe2x80x9cFiber Drawing and Strength Properties,xe2x80x9d Optical Fiber Communications, Vol. 1, Academic Press, Inc., 1995, at 179-248, the preform is generally arranged vertically in a draw tower such that a portion of the preform is lowered into a furnace region that typically heats the preform to temperatures around 2200xc2x0 C. The portion of the preform placed into the furnace region begins to soften, and the lower end of the preform forms what is known as the neck-down region, which is where the preform glass flows from the original cross-sectional area of the preform to the desired cross-sectional area of the fiber. From the lower tip of this neck-down region, the optical fiber is drawn.
One approach to preform manufacture involves the fabrication of an overcladding that surrounds an inner cladding and core. The overcladding does not have to meet some specifications as precisely as the core and inner cladding, and efforts to speed manufacture of preforms have therefore often focused on less expensive methods of forming the overcladding. One manner of forming the overcladding is the use of a sol-gel process. However, sol-gel methods have in the past tended to encounter cracking during the overcladding tube formation and subsequent drying process. Methods that suppressed such tendency included, for example, the use of supercritical drying and/or the use of drying control chemical additives (DCCA), both of which are relatively expensive and laborious. Other sol-gel processes have involved the precipitation of silica particles from solution. However, such precipitation processes typically involved the use of alkali silicates, and thus required further processing steps to remove the alkali metal ions.
Co-assigned U.S. Pat. No. 5,240,488 (the ""488 patent), the disclosure of which is hereby incorporated by reference, discloses a sol-gel process capable of producing crack-free overcladding preform tubes of a kilogram or larger. In this process, a colloidal silica dispersion, e.g., fumed silica, is obtained having a pH of 2 to 4. To obtain adequate stability of the dispersion and prevent agglomeration, the pH is raised to a value of about 9 to about 14 by use of a base. Typically, a commercially-obtained dispersion is pre-stabilized at such a pH value by addition of a base such as tetramethylammonium hydroxide (TMAH). Introduction of the TMAH raises the pH value. Other quaternary ammonium hydroxides behave similarly. When the pH is so raised, the silica takes on a negative surface charge due to ionization of silanol groups present on the silica surface. The negative charge of the silica particles creates mutual repulsion, preventing substantial agglomeration and maintaining the stability of the dispersion.
At a later stage in the process, as discussed in Col. 15, lines 39-65 of the ""488 patent, a gelling agent such as methyl formate is added to reduce the pH. The ester reacts to neutralize the base, and the negative character of the silica particles is thereby neutralized such that gelation is induced. Subsequent to gelation, the sol-gel body is typically released from its mold, and placed in an oven for drying and subsequent heat treatment, as reflected in the Table at Cols. 11-12 of the ""488 patent.
There exist a variety of methods for producing a silica sol useful in such sol-gel processes. See, e.g., U.S. Pat. Nos. 5,116,535 and 5,246,624. Another approach is presented in co-assigned U.S. patent application Ser. No. 08/971,460 (xe2x80x9cthe ""460 applicationxe2x80x9d), the disclosure of which is hereby incorporated by reference. In the ""460 application, the pH of a silica-water mixture is adjusted before or during mixing, resulting in destabilization of the mixture. Mixing within this unstable region provides a resultant sol with desirable stability.
While such processes generally provide useful results, improvements and refinements are continually sought.
The invention provides an improvement over current techniques for forming silica sols useful in optical fiber manufacture, including the technique of the ""460 application. The invention involves preparation of a silica sol having a desirable yield-dilatancy, such that commercial use of the sols is made easier and more attractive. Yield-dilatancy is characterized by an abrupt rise in a sol""s viscosity as shear rate increases, even to the point of gelation in some cases. Advantageously, therefore, sols exhibit this abrupt rise at high shear rates, i.e., the sols exhibit a high yield-dilatancy point. Use of sols having poor or unknown yield-dilatancy clearly introduces the danger of an abrupt viscosity rise during transport, e.g., pumping, through equipment in a fabrication plant, leading to clogging or similar problems that would cause a halt in production. Additionally, while those in the art have generally sought to move toward higher-loaded sols (e.g., at least 50 wt. % silica) to lower shrinkage and ease cracking in sol-gel bodies, it has been found that such higher-loaded sols tend to encounter more severe yield-dilatancy. This factor makes the invention even more significant for future commercial sol-gel processes.
According to the invention, silica sols exhibiting desirable yield-dilatancy are fabricated by the following process. A mixture comprising silica, water, and a pH-adjusting agent is provided, and the mixture is shear-mixed. Typically, at least a portion of the silica is added during the shear mixing, and, similarly, at least a portion of the pH-adjusting agent is typically added during or after the mixing. The desirable properties of the resultant sol are attained by using a concentration of pH-adjusting agent that moves the sol into a particular range of raised viscosity, and instability, during shear mixing.
The invention reflects the discovery, however, that the concentration of agent required to reach this range in fact changes depending on the properties of the silica mixture. And the invention further reflects a discovered technique for determining how to reach the raised viscosity range xe2x80x94by selecting the moles of pH-adjusting agent based on the silica surface area per unit volume of the mixture. The invention thus makes it possible to determine the appropriate concentration of pH-adjusting agent for varying types and amounts of silica. While previous techniques for fabricating silica sols may have been useful for the particular parameters disclosed therein, the invention makes it possible to produce high yield-dilatancy point sols for a variety of parameters, including higher-loaded sols.